Interpretive Summary: Global warming may result in hotter and drier conditions for growing plants on which we depend for food. We carried out this study to learn more about why plants do not grow as well under dry conditions. We found that growth is reduced because plants leaves become overly sensitive to carbon dioxide in the air. This increased sensitivity doesn't let as much carbon dioxide into the leaf where it can be converted to sugars that eventually become food. This information may help scientists develop plants that can still grow well, even when it is dry, by making them less sensitive to carbon dioxide.

Technical Abstract:
Leaf water potentials below threshold values result in reduced stomatal conductance. Stomatal closure at low leaf water potentials may serve to protect against cavitation of xylem. Possible control of stomatal conductance by leaf water potential or hydraulic conductance was tested by drying the rooting medium in four herbaceous annual species until stomatal conductance was reduced, and then lowering the carbon dioxide concentration to determine whether stomatal conductance and transpiration rate could be increased and leaf water potential decreased, and whether hydraulic conductance was reduced at the resulting lower leaf water potential. In all species, low carbon dioxide concentrations could reverse the stomatal closure due to drying despite further reductions in leaf water potential, and the resulting lower leaf water potentials did not result in reductions in hydraulic conductance. Complete reopening of stomata could be induced by low carbon dioxide concentrations in leaves in which stomatal conductance at air levels of carbon dioxide were as low as about 50% of maximum. The relative sensitivity of stomatal conductance to internal carbon dioxide concentration in the leaves of dry plants of each species averaged 3 to 4 times higher than in leaves of wet plants.